A CBOR Tag for Unprotected CWT Claims Sets
draft-birkholz-rats-uccs-03

Document Type Active Internet-Draft (individual)
Authors Henk Birkholz  , Jeremy O'Donoghue  , Nancy Cam-Winget  , Carsten Bormann 
Last updated 2021-03-08
Replaced by draft-ietf-rats-uccs, draft-ietf-rats-uccs
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RATS Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Standards Track                           J. O'Donoghue
Expires: 9 September 2021                     Qualcomm Technologies Inc.
                                                           N. Cam-Winget
                                                           Cisco Systems
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                            8 March 2021

               A CBOR Tag for Unprotected CWT Claims Sets
                      draft-birkholz-rats-uccs-03

Abstract

   CBOR Web Token (CWT, RFC 8392) Claims Sets sometimes do not need the
   protection afforded by wrapping them into COSE, as is required for a
   true CWT.  This specification defines a CBOR tag for such unprotected
   CWT Claims Sets (UCCS) and discusses conditions for its proper use.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 9 September 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components

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   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Motivation and Requirements . . . . . . . . . . . . . . . . .   3
   3.  Characteristics of a Secure Channel . . . . . . . . . . . . .   4
     3.1.  UCCS and Remote ATtestation procedureS (RATS) . . . . . .   4
     3.2.  Privacy Preserving Channels . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     5.1.  General Considerations  . . . . . . . . . . . . . . . . .   7
     5.2.  AES-CBC_MAC . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.4.  AES-CCM . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.5.  ChaCha20 and Poly1305 . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   A CBOR Web Token (CWT) as specified by [RFC8392] is always wrapped in
   a CBOR Object Signing and Encryption (COSE, [RFC8152]) envelope.
   COSE provides -- amongst other things -- the integrity protection
   mandated by RFC 8392 and optional encryption for CWTs.  Under the
   right circumstances, though, a signature providing proof for
   authenticity and integrity can be provided through the transfer
   protocol and thus omitted from the information in a CWT without
   compromising the intended goal of authenticity and integrity.  If a
   mutually Secured Channel is established between two remote peers, and
   if that Secure Channel provides the required properties (as discussed
   below), it is possible to omit the protection provided by COSE,
   creating a use case for unprotected CWT Claims Sets.  Similarly, if
   there is one-way authentication, the party that did not authenticate
   may be in a position to send authentication information through this
   channel that allows the already authenticated party to authenticate
   the other party.

   This specification allocates a CBOR tag to mark Unprotected CWT
   Claims Sets (UCCS) as such and discusses conditions for its proper
   use in the scope of Remote ATtestation procedureS (RATS) and the
   conveyance of Evidence from an Attester to a Verifier.

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   This specification does not change [RFC8392]: A true CWT does not
   make use of the tag allocated here; the UCCS tag is an alternative to
   using COSE protection and a CWT tag.  Consequently, in a well-defined
   scope, it might be acceptable to use the contents of a CWT without
   its COSE container and tag it with a UCCS CBOR tag for further
   processing -- or to use the contents of a UCCS CBOR tag for building
   a CWT to be signed by some entity that can vouch for those contents.

1.1.  Terminology

   The term Claim is used as in [RFC8725].

   The terms Claim Key, Claim Value, and CWT Claims Set are used as in
   [RFC8392].

   The terms Attester, Attesting Environment and Verifier are used as in
   [I-D.ietf-rats-architecture].

   UCCS:  Unprotected CWT Claims Set(s); CBOR map(s) of Claims as
      defined by the CWT Claims Registry that are composed of pairs of
      Claim Keys and Claim Values.

   Secure Channel:  A protected communication channel between two peers
      that can ensure the same qualities associated for UCCS conveyance
      as CWT conveyance without any additional protection.

   All terms referenced or defined in this section are capitalized in
   the remainder of this document.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Motivation and Requirements

   Use cases involving the conveyance of Claims, in particular, remote
   attestation procedures (RATS, see [I-D.ietf-rats-architecture])
   require a standardized data definition and encoding format that can
   be transferred and transported using different communication
   channels.  As these are Claims, [RFC8392] is a suitable format.
   However, the way these Claims are secured depends on the deployment,
   the security capabilities of the device, as well as their software
   stack.  For example, a Claim may be securely stored and conveyed
   using a device's Trusted Execution Environment (TEE, see
   [I-D.ietf-teep-architecture]) or especially in some resource
   constrained environments, the same process that provides the secure

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   communication transport is also the delegate to compose the Claim to
   be conveyed.  Whether it is a transfer or transport, a Secure Channel
   is presumed to be used for conveying such UCCS.  The following
   sections further describe the RATS usage scenario and corresponding
   requirements for UCCS deployment.

3.  Characteristics of a Secure Channel

   A Secure Channel for the conveyance of UCCS needs to provide the
   security properties that would otherwise be provided by COSE for a
   CWT.  In this regard, UCCS is similar in security considerations to
   JWTs [RFC8725] using the algorithm "none".  RFC 8725 states: "if a
   JWT is cryptographically protected end-to-end by a transport layer,
   such as TLS using cryptographically current algorithms, there may be
   no need to apply another layer of cryptographic protections to the
   JWT.  In such cases, the use of the "none" algorithm can be perfectly
   acceptable.".  Analogously, the considerations discussed in Sections
   2.1, 3.1, and 3.2 of RFC 8725 apply to the use of UCCS as elaborated
   on in this document.

   Secure Channels are often set up in a handshake protocol that
   mutually derives a session key, where the handshake protocol
   establishes the authenticity of one of both ends of the
   communication.  The session key can then be used to provide
   confidentiality and integrity of the transfer of information inside
   the Secure Channel.  A well-known example of a such a Secure Channel
   setup protocol is the TLS [RFC8446] handshake; the TLS record
   protocol can then be used for secure conveyance.

   As UCCS were initially created for use in Remote ATtestation
   procedureS (RATS) Secure Channels, the following subsection provides
   a discussion of their use in these channels.  Where other
   environments are intended to be used to convey UCCS, similar
   considerations need to be documented before UCCS can be used.

3.1.  UCCS and Remote ATtestation procedureS (RATS)

   For the purposes of this section, the Verifier is the receiver of the
   UCCS and the Attester is the provider of the UCCS.

   Secure Channels can be transient in nature.  For the purposes of this
   specification, the mechanisms used to establish a Secure Channel are
   out of scope.

   As a minimum requirement in the scope of RATS Claims, the Verifier
   MUST authenticate the Attester as part of the establishment of the
   Secure Channel.  Furthermore, the channel MUST provide integrity of
   the communication from the Attester to the Verifier.  If

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   confidentiality is also required, the receiving side needs to be be
   authenticated as well, i.e., the Verifier and the Attester SHOULD
   mutually authenticate when establishing the Secure Channel.

   The extent to which a Secure Channel can provide assurances that UCCS
   originate from a trustworthy attesting environment depends on the
   characteristics of both the cryptographic mechanisms used to
   establish the channel and the characteristics of the attesting
   environment itself.

   A Secure Channel established or maintained using weak cryptography
   may not provide the assurance required by a relying party of the
   authenticity and integrity of the UCCS.

   Ultimately, it is up to the Verifier's policy to determine whether to
   accept a UCCS from the Attester and to the type of Secure Channel it
   must negotiate.  While the security considerations of the
   cryptographic algorithms used are similar to COSE, the considerations
   of the secure channel should also adhere to the policy configured at
   each of the Attester and the Verifier.  However, the policy controls
   and definitions are out of scope for this document.

   Where the security assurance required of an attesting environment by
   a relying party requires it, the attesting environment may be
   implemented using techniques designed to provide enhanced protection
   from an attacker wishing to tamper with or forge UCCS.  A possible
   approach might be to implement the attesting environment in a
   hardened environment such as a TEE [I-D.ietf-teep-architecture] or a
   TPM [TPM2].

   When UCCS emerge from the Secure Channel and into the Verifier, the
   security properties of the Secure Channel no longer apply and UCCS
   have the same properties as any other unprotected data in the
   Verifier environment.  If the Verifier subsequently forwards UCCS,
   they are treated as though they originated within the Verifier.

   As with EATs nested in other EATs (Section 3.12.1.2 of
   [I-D.ietf-rats-eat]), the Secure Channel does not endorse fully
   formed CWTs transferred through it.  Effectively, the COSE envelope
   of a CWT shields the CWT Claims Set from the endorsement of the
   Secure Channel.  (Note that EAT might add a nested UCCS Claim, and
   this statement does not apply to UCCS nested into UCCS, only to fully
   formed CWTs)

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3.2.  Privacy Preserving Channels

   A Secure Channel which preserves the privacy of the Attester may
   provide security properties equivalent to COSE, but only inside the
   life-span of the session established.  In general, a Verifier cannot
   correlate UCCS received in different sessions from the same attesting
   environment based on the cryptographic mechanisms used when a privacy
   preserving Secure Channel is employed.

   In the case of a Remote Attestation, the attester must consider
   whether any UCCS it returns over a privacy preserving Secure Channel
   compromises the privacy in unacceptable ways.  As an example, the use
   of the EAT UEID [I-D.ietf-rats-eat] Claim in UCCS over a privacy
   preserving Secure Channel allows a verifier to correlate UCCS from a
   single attesting environment across many Secure Channel sessions.
   This may be acceptable in some use-cases (e.g. if the attesting
   environment is a physical sensor in a factory) and unacceptable in
   others (e.g. if the attesting environment is a device belonging to a
   child).

4.  IANA Considerations

   In the registry [IANA.cbor-tags], IANA is requested to allocate the
   tag in Table 1 from the FCFS space, with the present document as the
   specification reference.

       +========+===========+======================================+
       |    Tag | Data Item | Semantics                            |
       +========+===========+======================================+
       | TBD601 | map       | Unprotected CWT Claims Set [RFCthis] |
       +--------+-----------+--------------------------------------+

                          Table 1: Values for Tags

5.  Security Considerations

   The security considerations of [RFC7049] and [RFC8392] apply.

   Section 3 discusses security considerations for Secure Channels, in
   which UCCS might be used.  This documents provides the CBOR tag
   definition for UCCS and a discussion on security consideration for
   the use of UCCS in Remote ATtestation procedureS (RATS).  Uses of
   UCCS outside the scope of RATS are not covered by this document.  The
   UCCS specification - and the use of the UCCS CBOR tag,
   correspondingly - is not intended for use in a scope where a scope-
   specific security consideration discussion has not been conducted,
   vetted and approved for that use.

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5.1.  General Considerations

   Implementations of Secure Channels are often separate from the
   application logic that has security requirements on them.  Similar
   security considerations to those described in
   [I-D.ietf-cose-rfc8152bis-struct] for obtaining the required levels
   of assurance include:

   *  Implementations need to provide sufficient protection for private
      or secret key material used to establish or protect the Secure
      Channel.

   *  Using a key for more than one algorithm can leak information about
      the key and is not recommended.

   *  An algorithm used to establish or protect the Secure Channel may
      have limits on the number of times that a key can be used without
      leaking information about the key.

   The Verifier needs to ensure that the management of key material used
   establish or protect the Secure Channel is acceptable.  This may
   include factors such as:

   *  Ensuring that any permissions associated with key ownership are
      respected in the establishment of the Secure Channel.

   *  Cryptographic algorithms are used appropriately.

   *  Key material is used in accordance with any usage restrictions
      such as freshness or algorithm restrictions.

   *  Ensuring that appropriate protections are in place to address
      potential traffic analysis attacks.

5.2.  AES-CBC_MAC

   *  A given key should only be used for messages of fixed or known
      length.

   *  Different keys should be used for authentication and encryption
      operations.

   *  A mechanism to ensure that IV cannot be modified is required.

   [I-D.ietf-cose-rfc8152bis-algs], Section 3.2.1 contains a detailed
   explanation of these considerations.

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5.3.  AES-GCM

   *  The key and nonce pair are unique for every encrypted message.

   *  The maximum number of messages to be encrypted for a given key is
      not exceeded.

   [I-D.ietf-cose-rfc8152bis-algs], Section 4.1.1 contains a detailed
   explanation of these considerations.

5.4.  AES-CCM

   *  The key and nonce pair are unique for every encrypted message.

   *  The maximum number of messages to be encrypted for a given block
      cipher is not exceeded.

   *  The number of messages both successfully and unsuccessfully
      decrypted is used to determine when rekeying is required.

   [I-D.ietf-cose-rfc8152bis-algs], Section 4.2.1 constains a detailed
   explanation of these considerations.

5.5.  ChaCha20 and Poly1305

   *  The nonce is unique for every encrypted message.

   *  The number of messages both successfully and unsuccessfully
      decrypted is used to determine when rekeying is required.

   [I-D.ietf-cose-rfc8152bis-algs], Section 4.3.1 contains a detailed
   explanation of these considerations.

6.  References

6.1.  Normative References

   [IANA.cbor-tags]
              IANA, "Concise Binary Object Representation (CBOR) Tags",
              <http://www.iana.org/assignments/cbor-tags>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8725]  Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
              Current Practices", BCP 225, RFC 8725,
              DOI 10.17487/RFC8725, February 2020,
              <https://www.rfc-editor.org/info/rfc8725>.

   [TPM2]     "Trusted Platform Module Library Specification, Family
              "2.0", Level 00, Revision 01.59 ed., Trusted Computing
              Group", 2019.

6.2.  Informative References

   [I-D.ietf-cose-rfc8152bis-algs]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              rfc8152bis-algs-12.txt>.

   [I-D.ietf-cose-rfc8152bis-struct]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              rfc8152bis-struct-15.txt>.

   [I-D.ietf-rats-architecture]
              Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote Attestation Procedures Architecture", Work

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              in Progress, Internet-Draft, draft-ietf-rats-architecture-
              10, 9 February 2021, <https://www.ietf.org/archive/id/
              draft-ietf-rats-architecture-10.txt>.

   [I-D.ietf-rats-eat]
              Mandyam, G., Lundblade, L., Ballesteros, M., and J.
              O'Donoghue, "The Entity Attestation Token (EAT)", Work in
              Progress, Internet-Draft, draft-ietf-rats-eat-09, 7 March
              2021, <https://www.ietf.org/archive/id/draft-ietf-rats-
              eat-09.txt>.

   [I-D.ietf-teep-architecture]
              Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
              "Trusted Execution Environment Provisioning (TEEP)
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-teep-architecture-14, 22 February 2021,
              <https://www.ietf.org/archive/id/draft-ietf-teep-
              architecture-14.txt>.

Appendix A.  Example

   The example CWT Claims Set from Appendix A.1 of [RFC8392] can be
   turned into an UCCS by enclosing it with a tag number TBD601:

    <TBD601>(
      {
        / iss / 1: "coap://as.example.com",
        / sub / 2: "erikw",
        / aud / 3: "coap://light.example.com",
        / exp / 4: 1444064944,
        / nbf / 5: 1443944944,
        / iat / 6: 1443944944,
        / cti / 7: h'0b71'
      }
    )

Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt
   Germany

   Email: henk.birkholz@sit.fraunhofer.de

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   Jeremy O'Donoghue
   Qualcomm Technologies Inc.
   279 Farnborough Road
   Farnborough
   GU14 7LS
   United Kingdom

   Email: jodonogh@qti.qualcomm.com

   Nancy Cam-Winget
   Cisco Systems
   3550 Cisco Way
   San Jose, CA 95134
   United States of America

   Email: ncamwing@cisco.com

   Carsten Bormann
   Universitaet Bremen TZI
   Bibliothekstrasse 1
   28369 Bremen
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.de

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